1. Field of the Invention
The present invention relates to a manufacturing method of a package including a cavity for airtightly sealing an object to be housed, a package manufactured by this manufacturing method, and a piezoelectric vibrator, an oscillator, an electronic device and a radio timepiece that use this package.
2. Description of the Related Art
In recent years, mobile telephones and portable information terminal devices employ a piezoelectric vibrator that uses crystal or the like as a time source, a timing source of control signals or the like, and a reference signal source etc. As this type of piezoelectric vibrator, a structure is known that includes a base substrate (a base member), a piezoelectric vibrating reed (a crystal oscillator) mounted on the base substrate, and a lid substrate (a cap member) that is overlaid on the base substrate to airtightly seal the piezoelectric vibrating reed, as described in Patent Document 1 below, for example. Further, this piezoelectric vibrator includes external electrodes (external connection electrodes) on one surface of the base substrate and routing electrodes (crystal connection electrodes) on the other surface of the base substrate, and the piezoelectric vibrating reed is mounted on the routing electrodes. The external electrodes and the routing electrodes are electrically connected by a core portion (a metal member) that penetrates the base substrate.
Here, in Patent Document 1, in order to form the core portion when the above-described piezoelectric vibrator is manufactured, a method is adopted in which a through hole with a small diameter is opened in the base substrate, and a pin-shaped metal member is hammered in while the base substrate is in a heated and softened state. However, with this method, there is a possibility that a gap is formed between the core portion and the through hole. Therefore, it is difficult to ensure airtightness in a cavity formed between the base substrate and the lid substrate.
To address this, a method is conceivable in which, when the core portion is formed, a metal member is disposed in a through hole formed in the base substrate, and at the same time, a glass frit in a paste form is filled between the metal member and the through hole. The filled glass frit is fired to thereby integrally fix together with the through hole, the metal member and the glass frit. According to this method, it is possible to ensure airtightness in the cavity.
In Patent Document 1, when the core portion is formed, the pin-shaped metal member is used as described above. However, a method is also conceivable in which a rivet having a flat plate-shaped head portion and the core portion protruding from a rear surface of the head portion is used in place of the metal member. According to this method, the core portion can be disposed in the through hole by performing a simple operation of just inserting the core portion into the through hole until the rear surface of the head portion comes into contact with the surface of the base substrate. Further, after that, when the glass frit is filled in the through hole, the head portion of the rivet acts as a lid of the through hole, and it is also possible to suppress the glass frit from leaking to the outside.
Note that, when the core portion is formed using the rivet and the glass frit in this manner, generally after firing the glass frit, the head portion of the rivet is polished (ground down) and removed. This makes it possible for the base substrate, the glass frit and the core portion to be flush with each other, and each of the electrodes (the routing electrodes and the external electrodes) can be reliably formed on the base substrate.
Patent Document 1: JP-A-2002-124845
When the glass fit is fired when the core portion is formed, a binder contained in the glass frit is burned and gas is generated inside the glass frit. The gas is released to the outside through a section of the glass frit that is exposed to the outside. However, with the above-described known piezoelectric vibrator manufacturing method, since the head portion of the rivet acts as the lid of the through hole, there is a case in which the gas is not released and an air bubble is formed between the head portion of the rivet and the glass frit.
In this case, when the head portion of the rivet is removed after firing the glass frit, a recessed portion remains in the surface of the glass frit. When the routing electrodes, the external electrodes and the like are formed on the base substrate in a state in which the recessed portion remains in the glass frit, it is difficult to form each of the electrodes to have a uniform thickness, and there is a case in which the electrode is formed with a section whose thickness is considerably thin. In this case, there is a risk that the thin section formed in each of the electrodes is locally disconnected due to deterioration over time or the like, and there is a possibility that conductivity between the piezoelectric vibrating reed and the external electrodes is impaired.
Further, even when the head portion of the rivet is not removed after firing the glass frit, there is a risk that a crack or chip is generated due to the air bubble formed inside the glass frit.